Most of plastic wastes have conventionally been treated by landfill or incineration, and not been effectively used as resources. In landfill, there are such problems as difficulties in finding sites for landfill and destabilization at the ground of landfill. In incineration, there are such problems as a deterioration of the furnace, a generation of hazardous gases and offensive odors, and an emission of CO2.
In Japan, the Law for Promotion of Sorted Collection and Recycling of Containers and Packaging was enacted in 1995, and therefore the recovery and recycling of plastic wastes has been legally obligated. Additionally, other various laws concerning the recycling were enacted. Thus, the necessity of the recovery and recycling of plastic products increases.
In recent years, the decomposition of plastic wastes for recycling has been attempted to deal with these problems. JP-A-5-31000 discloses a process which comprises decomposing a plastic waste by a reaction using super- or subcritical water as a reaction media and recovering the decomposition product. JP-A-10-87872 discloses a process which comprises decomposing a plastic component of a fiber-reinforced plastic used in various structural materials with super- or subcritical water, and recovering the fiber such as a glass fiber or a carbon fiber for recycling.
In this point, it is difficult for a conventional process for decomposing a plastic using only sub- or supercritical water, to recover a monomer or oligomer reusable as a plastic raw material in high yield (see Comparative Example 1 described later). This appears to be caused by the fact that a monomer or oligomer obtained by the decomposition of a plastic (polymer) may be further decomposed by sub- or supercritical water, or the amount of a monomer or oligomer may be decreased by a side reaction. Therefore, the conventional plastic decomposition product obtained by sub- or supercritical water may be mainly recycled as a liquid fuel.
There is a need for a process for decomposing the plastic waste in which a monomer or oligomer can be recovered as a plastic raw material, and not as a liquid fuel in high yield in order to make effective use of petroleum resources.